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Ellipse :: Ellipse (n.) An oval or oblong figure, bounded by a regular curve, which corresponds to an oblique projection of a circle, or an oblique section of a cone through its opposite sides. The greatest diameter of the ellipse is the major axis, and the least diameter is the minor axis. See Conic section, under Conic, and cf. Focus..

Myopia :: Myopia (n.) Nearsightedness; shortsightedness; a condition of the eye in which the rays from distant object are brought to a focus before they reach the retina, and hence form an indistinct image; while the rays from very near objects are normally converged so as to produce a distinct image. It is corrected by the use of a concave lens..

Micrometer :: Micrometer (n.) An instrument, used with a telescope or microscope, for measuring minute distances, or the apparent diameters of objects which subtend minute angles. The measurement given directly is that of the image of the object formed at the focus of the object glass..

Focuses :: Focuses (pl. ) of Focu.

Hypermetropy :: Hypermetropy (n.) A condition of the eye in which, through shortness of the eyeball or fault of the refractive media, the rays of light come to a focus behind the retina; farsightedness; -- called also hyperopia. Cf. Emmetropia..

Directrix :: Directrix (n.) A straight line so situated with respect to a conic section that the distance of any point of the curve from it has a constant ratio to the distance of the same point from the focus.

Parabola :: Parabola (n.) A kind of curve; one of the conic sections formed by the intersection of the surface of a cone with a plane parallel to one of its sides. It is a curve, any point of which is equally distant from a fixed point, called the focus, and a fixed straight line, called the directrix. See Focus..

Concentre :: Concentre (v. t.) To draw or direct to a common center; to bring together at a focus or point, as two or more lines; to concentrate..

Focusing :: Focusing (p. pr. & vb. n.) of Focu.

Camera Obscura :: Camera obscura () An apparatus in which the images of external objects, formed by a convex lens or a concave mirror, are thrown on a paper or other white surface placed in the focus of the lens or mirror within a darkened chamber, or box, so that the outlines may be traced..

Diaphragm :: Diaphragm (n.) A plate with an opening, which is generally circular, used in instruments to cut off marginal portions of a beam of light, as at the focus of a telescope..

Image :: Image (n.) The figure or picture of any object formed at the focus of a lens or mirror, by rays of light from the several points of the object symmetrically refracted or reflected to corresponding points in such focus; this may be received on a screen, a photographic plate, or the retina of the eye, and viewed directly by the eye, or with an eyeglass, as in the telescope and microscope; the likeness of an object formed by reflection; as, to see one's image in a mirror..

Irradiation :: Irradiation (n.) The apparent enlargement of a bright object seen upon a dark ground, due to the fact that the portions of the retina around the image are stimulated by the intense light; as when a dark spot on a white ground appears smaller, or a white spot on a dark ground larger, than it really is, esp. when a little out of focus..

Reticule :: Reticule (n..) A system of wires or lines in the focus of a telescope or other instrument; a reticle.

Focal :: Focal (a.) Belonging to,or concerning, a focus; as, a focal point..

Collimator :: Collimator (n.) A tube having a convex lens at one end and at the other a small opening or slit which is at the principal focus of the lens, used for producing a beam of parallel rays; also, a lens so used..

Focalize :: Focalize (v. t.) To bring to a focus; to focus; to concentrate.

Aberration :: Aberration (n.) The convergence to different foci, by a lens or mirror, of rays of light emanating from one and the same point, or the deviation of such rays from a single focus; called spherical aberration, when due to the spherical form of the lens or mirror, such form giving different foci for central and marginal rays; and chromatic aberration, when due to different refrangibilities of the colored rays of the spectrum, those of each color having a distinct focus..

Focus :: Focus (n.) A point in which the rays of light meet, after being reflected or refrcted, and at which the image is formed; as, the focus of a lens or mirror..

Focimeter :: Focimeter (n.) An assisting instrument for focusing an object in or before a camera.

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